The relationship between electroretinography (ERG) data and the severity and prognosis of diabetic retinopathy will be determined. ERG data will be related to the severity of specific retinopathy features graded in stereo fundus photographs, and fluorescein angiograms. Data obtained in eyes with non-proliferative diabetic retinopathy will be used to determine whether the ERG can identify eyes at high risk to develop proliferative diabetic retinopathy (PDR). Data obtained in eyes prior to photocoagulation for (PDR) will be used to determine whether the ERG can identify eyes at high risk to respond poorly to photocoagulation treatment.